Interfacial Acid-Base Equilibria

Surface second harmonic generation (SSHG) is very nsefnl to measure the spectrum of interfacial species and therefore to do a pH titration in the interfacial layer that is not centrosymmetric. The concept of measnring a snrface pA j was introduced by Zhao et al. [40] who showed that the surface pA j of p-hexadecylaniline was 3.6 compared to a bulk value of 5.3, indicating that the interface prefers to accommodate neutral rather than charged species. 

In 1997, Tamburello et al. also measured the surface concentrations of different forms of eosin B at the air-water interface [41]. Two snrface p T values were measured to be 4.0 and 4.2, that is, larger values than the bulk values of 2.2 and 3.7, respectively. These shifts indicate that the nentral and the monoanionic forms of eosin B are favored at the interface compared to the monoanionic and the dianionic forms, respectively. 

In snmmary, spectroscopic studies of the properties of liquid-liquid interfaces have corroborated the conclusions drawn from molecular dynamics, namely, that the H2O-CCI4 interface is much sharper than the HjO-DCE interface. Additionally, it is clear that the interfacial polarity can be considered as the average of the polarities of the two solvents. Finally, it is worth pointing out that spectroscopic data can be directly compared to molecular dynamics calculations to extract structural information as recently reviewed by Benjamin [46]. 

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Petrov, J. G. Mobius, D. Interfacial acid-base equilibrium and electrostatic potentials of model Langmuir-Blodgett membranes in contact with phosphate buffer. Colloids Surf., A 2000, 171, 207-215. 

A merocyanine dye, l-ethyl-4-(2-(4-hydroxyphenyl)ethenyl)pyridinium bromide (M-Mc, 2), exhibits a large spectral change according to the acid-base equilibrium [40, 41]. The equilibrium is affected by the local electrostatic potential and the polarity of the microenvironment around the dye. Hence, this dye is useful as a sensitive optical probe for the interfacial potential and polarity when it is covalently attached to the polyelectrolyte backbone. 

Specific interactions in binary blends of ethylene-vinyl acetate copolymer with various low molecular weight terpene-phenol tackifying resins (TPR) were systematically investigated, as a function of the composition of the blend and of the electron acceptor ability of the resin, by using attenuated total reflection FTIR spectroscopy. Molecular acid-base were evidenced between TPR hydroxyl groups and EVA carbonyl groups. Quantitative information on the fraction of acid-base bonded entities, the enthalpy and equilibrium constant of pair formation were obtained. A crystalline transition of the EVA copolymer was observed and discussed in terms of enthalpy and entropy considerations based on FTIR and calorimetric DSC investigations. Fundamental results are then summarised to predict the interfacial reactivity of such polymer blends towards acid or basic substrates. 16 refs. 

The MEMED technique has been used to study the hydrolysis of aliphatic acid chlorides in a water/l,2-dichloroethane (DCE) solvent system [3]. It was shown unambiguously that the reaction proceeds via an interfacial process and shows saturation kinetics as the concentration of acid chloride in the DCE increases the data were well fitted to a model based on a pre-equilibrium involving Langmuir adsorption at the interface. First-order rate constants for interfacial solvolysis of CH3(CH2) COCl were 300 150(n = 2), 200 100(n = 3) and 120 60 s-1( = 8). 

For reversible adsorption the differential heats can be obtained by slicing the heat evolution peak obtained by use of an extended saturation of the adsorbent with a solution establishing a state of interfacial equilibrium. This is exemplified by the work of F. Fowkes and T. Lloyd who applied this technique to the adsorption of acids and bases on metal oxide pigments [25]. 

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